1. Field of the Invention
This invention relates to the field of screws that are used, for example, to melt or soften polymer, such as in a machine for injection molding polymer or a machine for extruding polymer.
2. Description of the Related Art
The use of screws to injection mold or extrude polymer is well known. Turning to FIG. 1, there is shown a conventional or standard screw 11 for use in injection molding which includes three zones: a feeding zone 13, a compression or transition zone 15 and a metering zone 17. Screw 11 is housed in a hollow cylindrical barrel 19 having a constant inner diameter and preferably a smooth inner surface. Polymer resin, which may be in any form such as pellets, granules, flakes or powder, is fed through an opening 21 in barrel 19 into feeding zone 13 where screw 11 turns to pack and then push the pellets into compression zone 15. The pellets are melted in compression zone 15 and then pushed to metering zone 17 where the molten material is homogenized. Afterwards the homogenized melt is either injection molded or processed further.
Screw 11 has a screw shaft 23 having a thread 25 spirally positioned about shaft 23 to form flights 25. Flights 25 are characterized by their depth, which is the height of flight 25 above shaft 23 and by their pitch, which is the length P of the distance between two adjacent flights 25 plus one flight width. The outside diameter OD of a screw 11 includes the depth of a flight 25 above and below shaft 23, whereas the root diameter RD of screw 11 is the diameter of shaft 23 only, without including the depth of flights 25. Conventionally flights 25 in a screw 11 have the same pitch in each of feeding zone 13, compression zone 15 and metering zone 17, but have a changing depth from zone to zone. Specifically, flights 25 have a constant depth x in feeding zone 13, a constant depth y in metering zone 17 where y<x, and a gradually decreasing depth of x to y in compression zone 15.
Screws are often characterized by their compression ratio, which is a ratio that is used to quantify the amount the screw compresses or squeezes the resin. The concept behind the compression ratio is to divide the volume of a flight in the feed section by the volume of a flight in the metering section, but the actual standard that is used is a simplified method based on the following equation:
      Compression    ⁢                  ⁢    ratio    =            depth      ⁢                          ⁢      of      ⁢                          ⁢      flight      ⁢                          ⁢      in      ⁢                          ⁢      feeding      ⁢                          ⁢      zone              depth      ⁢                          ⁢      of      ⁢                          ⁢      flight      ⁢                          ⁢      in      ⁢                          ⁢      metering      ⁢                          ⁢      zone      
This compression ratio is referred to as the depth compression ratio. High compression screws, which are usually used for crystalline or semi-crystalline materials, such as polymers, have compression ratios of greater than about 2.5. Standard compression screws, which are usually used for amorphous materials, have compression ratios of from about 1.8 to about 2.5, more commonly 2.2.
Various problems with high compression screws include: overheating caused by compression that is too high or is uncontrolled; “bridging”, which is when the polymer melt turns with the screw and is not pushed forward; and screw deposit which builds up in the compression and metering zones. These problems limit the maximum screw rotation speed and by consequence the output of molten material. In an attempt to overcome these problems some users switch to standard screws, but the depth of the flight in the metering zone of a standard screw is too high to give good melt homogeneity under some conditions, especially with crystalline materials.
Many attempts have been made to improve the performance of screws. U.S. Pat. No. 4,129,386, discloses an extrusion device which has a grooved barrel in combination with a screw having a helix angle or pitch D in the feed zone that constantly increases through a transition zone to a helix angle F in the metering zone. The feed zone has a constant flight height G, the metering zone has a constant flight I, and the t transition zone has a constantly decreasing flight height from feed zone flight height G to metering zone flight height I. This screw design suffers from problems of overfeeding of the material to be extruded, and requires a grooved barrel in order to prevent buildup of excessive pressure gradients along the screw.
What is needed, therefore, is a screw which will produce a homogeneous melt without the problems associated with screws having a high compression ratio.